Freight Train
This a fantastic book. It’s so simple, but I think that's what makes it so fantastic. While on the surface it’s about a freight train that takes a trip, it’s actually about teaching the younger side of little ones colors (though there are a few other concepts like light and dark.)
Couldn’t recommend this one more for little little ones. We’ve owned this one for several years and still read it for entertainment. We actually love this one so much we just donated a whole bunch of these to a non-profit that gives little ones in foster care support.
Cautions: Absolutely none.